A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Control systems using feedforward may be used to control relative movement of various components of the lithographic apparatus. For example, such a system may be used to control the motion of a substrate table based on feedforward of a measurement of the acceleration of a projection system (which may vibrate due to noise excitation for example). As a further example, a feedforward control system may be used to control the relative motion of a reticle and the substrate table. The performance of such systems may be limited due to delays in the feedforward branch, or, more generally, time lags between a reaction of a controlled component to the feedforward and a corresponding change in the reference signal.